Generating a composite signal with code division multiplexing (CDM) and time division multiplexing (TDM) pilots

ABSTRACT

A composite signal may be generated with code division multiplexing (CDM) and time division multiplexing (TDM) pilots. A first pilot sequence may be a CDM pilot sequence. A second pilot sequence may have TDM pilot sequences including a cyclic prefix. The composite signal may have a cyclic prefix time domain portion based on the cyclic prefix and a base portion of the composite signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of InternationalApplication No. PCT/EP2009/008474,filed Nov. 27, 2009, claiming priorityto Great Britain Application No. 0821745.7, filed Nov. 27, 2008, both ofwhich are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to employing a pilot transmission scheme in acommunication system and in particular, but not exclusively, toemploying a pilot transmission scheme in a broadcast 3^(rd) GenerationPartnership Project (3GPP) cellular communication system.

BACKGROUND OF THE INVENTION

Currently, 3rd generation cellular communication systems are beingrolled out to further enhance the communication services provided tomobile phone users. The most widely adopted 3rd generation communicationsystems are based on Code Division Multiple Access (CDMA) and FrequencyDivision Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMAsystems, user separation is obtained by allocating different spreadingand/or scrambling codes to different users on the same carrier frequencyand in the same time intervals. There may also be a time divisionmultiple access (TDMA) component in CDMA systems, where user separationis also achieved by assigning different time slots to different users.

In FDD systems, uplink and downlink communication occur on separatecarriers. Uplink transmissions are those from the mobile wirelesscommunication unit (often referred to as wireless subscribercommunication unit) to the communication infrastructure via a wirelessserving base station. Downlink transmissions are those from thecommunication infrastructure to the mobile wireless communication unitvia a serving base station. In contrast to FDD systems, TDD systems usethe same carrier frequency for both uplink and downlink transmissions.In both FDD and TDD systems, the carrier frequency may be subdivided inthe time domain into a series of timeslots in order to provide a TDMAcomponent. For TDD, the single carrier frequency is assigned to uplinktransmissions during some timeslots and to downlink transmissions duringother timeslots. For FDD, a carrier frequency operable in either uplinkor downlink mode may service different users during different timeregions, which may comprise one or more timeslots. An example of acommunication system using this principle is the Universal MobileTelecommunication System (UMTS). Further description of CDMA, andspecifically of the Wideband CDMA (WCDMA) mode of UMTS, can be found in‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley &Sons, 2001, ISBN 0471486876.

In a conventional cellular system, cells in close proximity to eachother are allocated non-overlapping transmission resources. For example,in a CDMA network, cells within close proximity to each other areallocated distinct spreading codes (to be used in both the uplinkdirection and the downlink direction). This may be achieved by, forexample, employing the same channelisation spreading codes at each cell,but a different cell specific scrambling code. The combination of theseleads to effectively distinct spreading codes at each cell.

A typical and most cost-effective approach in the provision ofmultimedia services is to ‘broadcast’ (point-to-multipoint transmission)the multimedia signals, as opposed to sending the multimedia signals inan uni-cast (i.e. point-to-point) manner. For broadcastpoint-to-multipoint transmission, a single carrier frequency conveysbroadcast information to a plurality of wireless subscriber units fromone wireless serving communication unit, i.e. one cell. Typically, tensof channels carrying say, news, movies, sports, etc. may be broadcastsimultaneously over such a communication network. Conversely, foruni-cast operation, the communication is on a one-to-one basis between awireless subscriber unit and a serving wireless communication unit, i.e.the information conveyed is unique to one wireless subscriber unit.

In some cases, an entire carrier may be dedicated to the sending ofbroadcast or point-to-multipoint information. The broadcast carrier maybe associated with one or more uni-cast communication channels, whichmay be operable over one or more separate carrier frequencies.Additionally, it is possible for both uni-cast and point-to-multipointbroadcast traffic to be conveyed over the same carrier frequency buttypically at different times. In general, uni-cast communication mayallow for the establishment of security and authentication mechanismsrelated to the communication of the broadcast information between awireless subscriber unit and a communication network, and may alsofacilitate the transfer of broadcast service information to the wirelesssubscriber unit. Other user-specific communication may be performed overthe uni-cast carrier(s), which may or may not relate to the operation ofbroadcast services on the same or another carrier frequency.

Digital communication systems may use so-called non-coherent or coherentsignalling methods. For either method, it is common that thetransmitting entity maps the desired bit sequence for transmission ontoa sequence of modulation symbols, each adopting one of a finite alphabetof symbols or waveforms. As the signals propagate from the transmitterto the receiver, the phase of the transmitted signal varies in space andin time. Generally, at a receiver, the phase of the received signal isarbitrary.

In the non-coherent method, the receiver does not require knowledge ofthe phase of the received signal in order to demodulate the signal andto recover the transmitted data. That is, the members of the transmittedsymbol alphabet for the non-coherent modulation scheme may bedistinguished from one another by the receiver without the need forabsolute phase information.

Conversely, for coherent modulation schemes, members of the transmittedsymbol alphabet may appear similar to one another at different phases.Thus, for these schemes, it is imperative that the receiver is able todetermine the received phase of the signal in order to distinguishbetween the received symbols and to correctly recover the data. In manycircumstances, coherent modulation schemes are able to carry spectralefficiency advantages. Hence, coherent modulation schemes are commonlyused for high-capacity digital communication and broadcast systems.

In the coherent scheme, the transmitter often sends a reference signalalong with the transmitted data. The receiver has a-priori knowledge ofthe structure of this reference signal. Hence, the receiver is able tolook for the presence of the reference signal within the receivedsignal. Upon finding the reference signal, the receiver may determineits amplitude and phase and, assuming that both the reference signal andthe communication data have passed through the same propagation channelbetween transmitter and receiver, the phase of the additionally receivedcommunication data symbols is then also known and the modulation symbolsmay be recovered. The process of estimating the amplitude and phase ofthe radio propagation channel in the receiver is known as ‘channelestimation’.

The reference signal is often referred to as a ‘pilot’. At thetransmitter, the pilot must be multiplexed with the data in some waysuch that both may then be carried over the communication link to thereceiver, with the intention of ensuring that both will experience thesame or similar phase adjustments by the time that the signals arrive atthe receiver. Code-Division Multiplexing (CDM), Frequency-DivisionMultiplexing (FDM) and Time-Division Multiplexing (TDM) methods are eachindividually used in various communication systems to transmit pilotsignals as well as data.

FIG. 1 illustrates examples of these pilot/data multiplexingpossibilities. For example, a first graph 100 illustrates a CDMtechnique with the code values 115 plotted against time. Here, it isshown that the data is sent using a first set of codes and the pilotsignal is sent using a second or second set of codes. A second graph 150illustrates a TDM technique with a code or frequency 155 plotted againsttime. Here, the data 165 is sent in a first time period with the pilotsignal 170 sent in a second time period.

The current UMTS WCDMA FDD system utilises CDM between the pilot signaland the data. The pilot is termed the Common Pilot Channel (CPICH). TheCPICH is designed such that it is orthogonal in the code domain to thedata. This helps to reduce interference between the data and pilotsignal, which is beneficial in terms of receiver performance. Thepresence of code-domain orthogonality between the pilot signal and datahelps to avoid the possibility of the data signal interfering with thepilot, which would otherwise reduce the quality of the estimate of theamplitude and phase of the pilot. This means that the characteristics ofthe radio propagation channel can be better-ascertained by the receiverand demodulation performance is improved (such as through a reducednumber of communication errors, improved geographical coverage of thesystem, improved communication data rates, etc.).

The code-domain orthogonality between the pilot and data is present atthe transmitting side, but can sometimes be degraded or destroyed by thetime that the signals arrive at the receiver. This degradation is oftendue to the action of the intervening radio propagation channel. Inparticular, radio channels with a large amount of signal dispersion maysignificantly degrade the degree of orthogonality between pilot and datausing a CDM technique. An example of this signal dispersion (a spreadingin time of the signal energy due to multiple reflections and thediffering path lengths of individual propagation rays) is illustrated inthe time dispersion graphical representation 200 of FIG. 2. Thus, asshown, in some radio environments, a code domain pilot is susceptible tothe radio propagation channel and exhibits a variable channel amplitude(and also phase—not explicitly shown) response 210 over time 215. Assuch, the use of CDM pilot signals can be less effective than would bedesirable.

In such scenarios, it can be beneficial to alternatively utilise timedivision multiplexing for the pilot signal and data. Orthogonalitybetween the pilot signal and the data is again susceptible todegradation due to the overlap of energy between the two, which iscaused by the delay spread in the radio propagation channel. Referringback to FIG. 1, the time dispersion of data 175 in the radio channel maylead to a time domain overlap of energy between data and pilot signals.In the subsequent region 180, the pilot signal is not affected by thedata even in the presence of dispersion, due to the time limitation ofthe imposed dispersion. Hence, the quality of the channel estimation isnot degraded if this portion of the pilot signal is used for channelestimation.

Therefore, by allowing for some guard separation in the time domainbetween the pilot signal and data, or by careful design of the TDM pilotsequence, it is still possible to receive a portion of the TDM pilotthat is unaffected by the data (and vice versa). Such careful design mayensure accurate estimation of the amplitude and phase of the radiochannel using the TDM pilot as well as deliver an improved demodulationperformance. As previously stated, these improvements in demodulationperformance may be translated into system gains such as improvedgeographical coverage or increased data rates.

Large amounts of signal dispersion can occur due to the presence ofmultiple reflections in the radio channel. Larger differential pathdelays lead to a larger extent in time of the dispersion whilst thepresence of multiple reflectors leads to an increased number ofcomponents (more paths). Such channels are referred to herein as‘complex’ radio channels in that they may exhibit a large number ofreflections.

One particular scenario where complex radio channels may be observed isthat of the Single Frequency Network (SFN) transmission method forbroadcast. In this transmission method, the same data is transmittedusing the same signal waveforms from multiple transmission sites (i.e.multiple communication units) in a synchronised manner. The waveformstravel towards the (potentially mobile) receiver (i.e. wirelesssubscriber unit) and experience differing delays and amplitude and phaseadjustments as they do so. The signals combine in space sometimesconstructively and sometimes destructively. The presence of differingsignal delays can allow for signals with a path delay difference to beresolved and constructively combined by the receiver. This process isoften referred to as equalisation. Accurate channel estimation istherefore essential in such systems and environments to enable theconstructive combination of the signals from the multiple transmissionsites. In the absence of equalisation, the presence of multiple pathreflections may severely degrade the radio link quality.

Therefore, it has been determined that the use of a CDM pilot forsystems (such as the aforementioned SFN broadcast system) may not beoverly appropriate in complex propagation channels. The use of a TDMpilot may offer advantages in terms of channel estimation and receiverperformance. However, many 3GPP receivers are designed to operate usingCDM pilots (and may also use the pilot for purposes other than channelestimation in the receiver).

Consequently, current techniques are suboptimal. Hence, an improvedmechanism to address the problem of pilot transmission schemes, forexample over a broadcast cellular network, would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the invention seeks to mitigate, alleviate or eliminate oneor more of the abovementioned disadvantages singly or in anycombination.

According to aspects of the invention, there is provided, a cellularcommunication system, integrated circuits and communication unitsarranged to perform a methods of utilising a pilot transmission schemein accordance with the concepts herein described.

In one aspect of the invention, a method of pilot-assisted datacommunication is described. The method may be applied to pilot-assisteddata communication generated within one or more time slots used for datatransmission. The method comprises transmitting a composite signal,where the composite signal comprises data, a first pilot sequence, suchas a CDM pilot sequence, with non-cyclic properties that is transmittedover a designated or pre-defined time region of the one or more timeslots and a second pilot sequence, for example a TDM pilot sequence,where the first pilot sequence and second pilot sequence are formedwithin the composite signal in substantially the same pre-defined timeregion of the one or more time slot. The second pilot sequence isarranged such that the composite signal exhibits a cyclic prefixedstructure. In one optional embodiment the substantially simultaneoustransmission of the first pilot sequence and second pilot sequencecomprises overlapping time periods within the pre-defined time region.

In one optional embodiment the composite signal may be transmittedduring a first prefix segment of the pre-defined time region may besubstantially the same as the composite signal transmitted during asecond base segment of the pre-defined time region. For example, in oneoptional embodiment, the first prefix segment may be copied from thesecond base segment.

In one optional embodiment the first pilot sequence may be code-domainorthogonal to the second pilot sequence.

In one optional embodiment a non-cyclic scrambling operation may beperformed on the first pilot sequence and/or the second pilot sequenceduring the pre-defined time region.

In one optional embodiment the second pilot sequence may be comprised ofa combination of pilot sequence fragments within a multiplexing logicunit.

In one optional embodiment the multiplexing logic unit may comprise theoperations of CDMA spreading and/or code multiplexing.

In one optional embodiment the multiplexing logic unit comprises afrequency domain multiplexing operation.

In one optional embodiment the second pilot sequence may be constructedusing a finite symbol alphabet, such as QPSK, 16-QAM, 64-QAM.

In one optional embodiment the second pilot sequence may be constructedusing constant-amplitude symbols.

In one optional embodiment the first pilot sequence may be a portion ofa common pilot channel of a 3GPP WCDMA UMTS system.

In one optional embodiment the second pilot sequence may be arranged inorder to result in a substantially flat frequency-domain composition ofthe second ‘base’ time segment of the pre-defined time region of thecomposite signal.

These and other aspects, features and advantages of the invention willbe apparent from, and elucidated with reference to, the embodimentsdescribed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the accompanying drawings, in which

FIG. 1 illustrates an effect of time dispersion on CDM and TDM pilotmultiplexing methods.

FIG. 2 illustrates the effect of time dispersion of a signal in a radiopropagation channel.

FIG. 3 illustrates a broadcast communication system adapted inaccordance with some embodiments of the invention.

FIG. 4 illustrates a communication unit adapted in accordance with someembodiments of the invention.

FIG. 5 illustrates data transmission using a combination of CDM and TDMpilots in accordance with some embodiments of the invention.

FIG. 6 illustrates a CDMA signal construction using spreading, codemultiplexing and scrambling in accordance with some embodiments of theinvention.

FIG. 7 illustrates an example of a cyclic construction of a compositesignal in a TDM pilot time region in accordance with one embodiment ofthe invention.

FIG. 8 illustrates an example of a creation of a cyclic pilot signalregion using a non-cyclic CDM pilot sequence and a non-cyclic TDM pilotsequence in accordance with one embodiment of the invention.

FIG. 9 illustrates examples of CDM and TDM pilot construction methods inaccordance with one embodiment of the invention.

FIG. 10 illustrates an example of a receiver processing operation(steps) using combined CDM and TDM pilot signals in accordance with oneembodiment of the invention.

FIG. 11 illustrates examples of embodiments of the invention whenapplied to adaptive equalizer architectures.

FIG. 12 illustrates an example of a sequence search process employed insome embodiments of the invention.

FIG. 13 illustrates a typical computing system that may be employed toimplement signal processing functionality in embodiments of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following description focuses on embodiments of the inventionapplicable to a UMTS (Universal Mobile Telecommunication System)cellular communication system and in particular to a UMTS TerrestrialRadio Access Network (UTRAN) operating in any unpaired spectrum within a3^(rd) generation partnership project (3GPP) system. However, it will beappreciated that embodiments of the invention are not limited to thisparticular cellular communication system, but may be applied to anycommunication system that utilizes or may be adapted to support a pilottransmission scheme.

Embodiments of the invention propose a pilot transmission scheme thatcomprises both a CDM pilot and a TDM pilot, with the system beingdesigned to benefit from use of both the CDM pilot component and the TDMpilot component. Thus, in some legacy-based CDMA communication systems,existing CDM pilot structures may be retained to minimise disturbance tothese parts of the design within a wireless subscriber unit or terminal.In addition, the ability to use legacy CDM pilot signals reduces a needto redesign parts of the receiver that manage CDM channel estimation.

Referring now to FIG. 3, a cellular-based communication system 300 isshown in outline, in accordance with one embodiment of the presentinvention. In this embodiment, the cellular-based communication system300 is compliant with, and contains network elements capable ofoperating over, an universal mobile telecommunication system (UMTS)air-interface. In particular, some embodiments relate to the ThirdGeneration Partnership Project (3GPP) specification for wide-bandcode-division multiple access (WCDMA), time-division code-divisionmultiple access (TD-CDMA) and time-division synchronous code-divisionmultiple access (TD-SCDMA) standard relating to the UTRAN radioInterface (described in the 3GPP TS 25.xxx series of specifications).

In particular, the 3GPP system 300 is adapted to support both broadcastand uni-cast UTRA communication from one or more cells.

A plurality of wireless subscriber communication units/terminals (oruser equipment (UE) in UMTS nomenclature) 314, 316 communicate overradio links 319, 320 with a plurality of base transceiver stations,referred to under UMTS terminology as Node-Bs, 324, 326. The systemcomprises many other UEs and Node-Bs, which for clarity purposes are notshown.

The wireless communication system, sometimes referred to as a NetworkOperator's Network Domain, is connected to an external network 334, forexample the Internet. The Network Operator's Network Domain includes:

(i) A core network, namely at least one Gateway General Packet RadioSystem (GPRS) Support Node (GGSN) (not shown) and at least one ServingGPRS Support Nodes (SGSN) 342, 344; and

(ii) An access network, namely:

(i) A UMTS Radio network controller (RNC) 336, 340; and

(ii) A UMTS Node-B 324, 326.

The GGSN (not shown) or SGSN 342, 344 is responsible for UMTSinterfacing with a Public network, for example a Public Switched DataNetwork (PSDN) (such as the Internet) 334 or a Public Switched TelephoneNetwork (PSTN). The SGSN 342, 344 performs a routing and tunnellingfunction for traffic, whilst a GGSN links to external packet networks.

The Node-Bs 324, 326 are connected to external networks, through RadioNetwork Controller stations (RNC), including the RNCs 336, 340 andmobile switching centres (MSCs), such as SGSN 344. A cellularcommunication system will typically have a large number of suchinfrastructure elements where, for clarity purposes, only a limitednumber are shown in FIG. 3.

Each Node-B 324, 326 contains one or more transceiver units andcommunicates with the rest of the cell-based system infrastructure viaan I_(ub) interface, as defined in the UMTS specification.

In accordance with embodiments of the invention, a first wirelessserving communication unit (e.g. Node-B 326) has been adapted tocomprise logic modules as shown in, and described with reference to,FIG. 4. In accordance with embodiments of the invention, a subscribercommunication unit, such as a UE 314, has also been adapted to compriselogic modules as shown in, and described with reference to, FIG. 4.

For completeness, it is noted that each RNC 336, 340 may control one ormore Node-Bs 324, 326. Each SGSN 342, 344 provide a gateway to theexternal network 334. The Operations and Management Centre (OMC) 346 isoperably connected to RNCs 336, 340 and Node-Bs 324, 326. The OMC 346comprises processing functions (not shown) and logic functionality 352in order to administer and manage sections of the cellular communicationsystem 300, as is understood by those skilled in the art.

Management logic 346 communicates with one or more RNCs 336, 340, whichin turn provide the signalling 358, 360 to the Node-Bs and to the UEsregarding radio bearer setup, i.e. those physical communicationresources that are to be used for broadcast and uni-cast transmissions.

The management logic 346 is operably coupled to broadcast mode logic350. The broadcast mode logic 350 comprises or is operably coupled tosignalling logic for signalling to the plurality of wireless subscribercommunication units that part or all of the transmission resource in thecellular communication system 300 is to be configured or re-configuredfor broadcast mode of operation.

The broadcast mode logic 350 is configured to manage the physicalresources that are signaled to the RNCs and the Node Bs. In this manner,the broadcast mode logic 350 allocates timeslots or carrier frequenciesfor broadcast, sets transit powers and allocates a cell IDs for alltimeslots or carrier frequencies that are to carry broadcasttransmissions.

Referring now to FIG. 4, a block diagram of a wireless communicationunit 400 is shown and adapted in accordance with some embodiments of theinvention. In practice, purely for the purposes of explainingembodiments of the invention, the wireless communication unit isdescribed in terms of either a NodeB implementation or a user equipment(UE) implementation, with the functional elements being similar. Thewireless communication unit 400 contains an antenna 402 coupled toantenna switch 404 that provides isolation between receive and transmitchains within the wireless communication unit 400.

The receiver chain, as known in the art, includes receiver front-endcircuitry 406 (effectively providing reception, filtering andintermediate or base-band frequency conversion). The front-end circuitry406 is serially coupled to a signal processing function 408. An outputfrom the signal processing function 408 is provided to a suitable outputdevice 410. A controller 414 maintains overall subscriber unit control.The controller 414 is also coupled to the receiver front-end circuitry406 and the signal processing function 408 (generally realised by adigital signal processor (DSP)). The controller is also coupled to amemory device 416 that selectively stores operating regimes, such asdecoding/encoding functions, synchronisation patterns, code sequences,direction of arrival of a received signal and the like.

In accordance with embodiments of the invention, the timer 418 isoperably coupled to the controller 414 to control the timing ofoperations (transmission or reception of time-dependent signals) withinthe wireless communication unit 400.

As regards the transmit chain, this essentially includes an input device420, such as a keypad, coupled in series through transmitter/modulationcircuitry 422 and a power amplifier 424 to the antenna 402. Thetransmitter/modulation circuitry 422 and the power amplifier 424 areoperationally responsive to the controller 414.

The signal processor 408 in the transmit chain may be implemented asdistinct from the signal processor in the receive chain. Alternatively,a single processor 408 may be used to implement processing of bothtransmit and receive signals, as shown in FIG. 4. Clearly, the variouscomponents within the wireless communication unit 400 can be realized indiscrete or integrated component form, with an ultimate structuretherefore being an application-specific or design selection.

In accordance with embodiments of the invention, the signal processor408 has been adapted to comprise logic (encompassing hardware, firmwareor software) for supporting a combined pilot transmission scheme,dependent upon whether the wireless communication unit 400 is either abase station, say in a form of a NodeB or a wireless subscribercommunication unit, say in a form of an UE.

For example, in a first embodiment, let us consider that the wirelesscommunication unit 400 is a Node B. In a Node-B context, the signalprocessor 408 may be adapted to generate a combined TOM pilot and CDMpilot dependent upon any number of ways to generate a TDM pilot andcombine the TDM pilot in a composite signal that includes a CDM pilotsequence. In this example embodiment, the signal processor 408 maycomprise modulator logic 436, for example if the TDM pilot isconstructed using the OVSF construction method (e.g. as described laterwith respect to FIG. 9D). In this example embodiment, a TDM pilot may begenerated and introduced into the transmission stream using an existingmodulator with a new set of modulated symbols (which may sometimeshereinafter be termed pilot sequence ‘fragments’). The new set ofmodulated symbols may be introduced at the input of modulator logic 436during a TDM pilot time region of one or more time slot, as willsubsequently become apparent in later described embodiments. Inembodiments of the invention, the composite signal is applied in asingle time slot. However, in alternative embodiments, it is envisagedthat the composite signal may be applied across multiple time slots. Inyet further alternative embodiments, it is also envisaged that one timeslot may be of variable length.

If the sequence conforms to existing modulation alphabets, it isenvisaged in one example embodiment that any existing modulator logic ina legacy NodeB may not need adapting. However, if, in one example, a TDMpilot uses a modulation alphabet that is inconsistent with themodulation alphabet used by the data, then the modulator logic 436 maybe replaced by symbol quantisation logic 437. The symbol quantisationlogic 437 may be arranged to handle a new modulation alphabet (e.g.using different symbol quantisation) and may be located prior to aseparate CDMA modulator (as shown in FIG. 9D) or prior to multiplexinglogic (as shown in FIG. 9C). The symbol quantisation logic 437 may alsobe inserted in TDM pilot signal paths (pre scrambling, if applicable andas illustrated in FIG. 9A and FIG. 9B).

Furthermore, the NodeB signal processor may comprise adapted pilotgeneration logic 438. The pilot generation logic 438 may be arranged tobe a separate logical entity handling generation of pilot waveforms(e.g. not using existing modulator logic). For example, in oneembodiment, the pilot generation logic 438 may have been configured tohandle the generation of a CDM pilot (e.g. a CPICH in a 3GPP system) ina legacy NodeB. In the adapted NodeB a TDM pilot may be introduced in afirst example embodiment using a separate pilot generation logic (notshown) arranged to operate in parallel with distinct CDM pilotgeneration logic and arranged to generate the TDM pilot portion of thedata transmission. In this embodiment, the outputs of the two pilotgeneration logic units would be summed. Alternatively, in a secondexample embodiment, an existing pilot generation logic/unit could bemodified to generate a single combined CDM/TDM pilot waveform.

In a second embodiment, let us consider that the wireless communicationunit 400 is a UE. In a UE context, the signal processor 408 may beadapted to generate a combined TDM pilot and CDM pilot dependent upon anumber of ways to generate the TDM pilot. In one example embodiment, thesignal processor 408 may comprise channel estimator logic 430. In anapplication that uses legacy UEs, there would generally be an existingchannel estimator that is based around estimation using the CDM pilot(e.g. a CPICH in a 3GPP system). In a second UE-based embodiment, a TDMpilot component may alternatively be instantiated using a new TDM-pilotchannel estimation unit, and optionally combine the TDM pilot and CDMpilot outputs to provide an improved combined channel estimate, asillustrated and described further with reference to FIG. 11.

Alternatively, in the second UE-based embodiment, an existing CDM pilotchannel estimator may be modified to create a new single channelestimation logic/unit 432 that operates on all relevant parts of thereceived signal at its input. The derived channel estimates may beapplied to an equalisation and data recovery block, for example asillustrated further with reference to FIG. 11.

Although embodiments of the invention are described with regard to abroadcast system, whereby a NodeB transmitter is adapted to transmit atransmission using a combined CDM pilot and TDM pilot, and a UE receiveris adapted to receive and process a transmission that uses a combinedCDM pilot and TDM pilot, it is envisaged that the embodiments hereindescribed are equally applicable to a uni-cast (point-to-point) system.Here, a UE may be additionally adapted to support a transmission using acombined CDM pilot and TDM pilot, with a NodeB receiver also beingadapted to receive and process a transmission that uses a combined CDMpilot and TDM pilot from a transmitting UE.

In an alternative embodiment to the above direct channel estimationapproach, it is envisaged that the TDM pilot and CDM pilot signals maybe provided as inputs to adaptively-trained equaliser logic 434. In thisalternative embodiment, which is further described in FIG. 11, theadaptively-trained equaliser logic 434 would previously have taken theknown local replica of the CDM pilot as its input, to which theequalised pilot output of the adaptively-trained equaliser logic 434 iscompared to form an error signal that drives the feedback viacoefficient adaptation logic (not shown). When the TDM pilot is added,it is envisaged in one example embodiment that one adaptation logic unitmay be retained (as there is still only one equaliser logic unit).However, it is also envisaged that the adaptation logic unit may bedriven by a single error signal, or by two separate error signals.

In an embodiment where there are separate error signals, one errorsignal may be formed using a TDM pilot local replica, and one using aCDM pilot local replica. Pilot demultiplexing logic (not shown) may beused to separate the received and equalised pilots from the compositeequalised signal. The pilot demultiplexing logic would typically consistof a matched filter (matched to the pilot), for example, or logic thatextracts a particular pilot region of the received output.

In an embodiment that uses separate error signals, the pilotdemultiplexing logic may be arranged to isolate the CDM portion and theTDM portion from the separate error signals respectively. Each of theseseparate outputs may be compared against their respective localreplica's forming two error signals. In this example, these two errorsignals may be fed to the (single) adaptation logic that is arranged tothen calculate new coefficients and apply them to the equaliser logic.

In an embodiment where there is a single error signal, the pilotdemultiplexing logic may be arranged to produce a single compositeoutput that is matched to the composite (TDM and CDM) equalised pilotsignals. This may be compared against a composite (TDM and CDM) localreplica and the resultant single error signal may be used to drive theequaliser adaptation process.

In a yet further embodiment of the invention, the channel estimationtechniques described above may be combined with adaptive equalisertechniques. For example, it is envisaged that an adaptive equalisertechnique may be used for the CDM pilot and the coefficient adaptationprocess may take account of an auxiliary channel estimate formed usingthe TDM pilot.

Embodiments of the invention provide a data transmission 500 thatcombines a CDM pilot waveform 535 and a TDM pilot waveform 530 thatoverlap in time, as shown in FIG. 5. The data transmission 500 isillustrated as a representation of channelisation codes 510 versus time515. Here, the data 525 and TDM pilot sequence 530 share the samechannelisation codes, with the CDM pilot 535 (for example on a CPICH)utilising a different set of channelisation codes. Together, the data525, TDM pilot sequence 530 and CDM pilot 535 form a composite sequencesignal.

In some embodiments of the invention, the TDM pilot 530 may or may notbe orthogonal in code to the CDM pilot 535, although in someapplications there may be some advantages in provision of thisorthogonal property. Data 525 is transmitted in units of a time slot520, in which a CDM pilot 535 and a TDM pilot 530 instance arecontained. In general the construction of the CDM pilot waveforms 535and TDM pilot waveforms 530 may vary from time slot 520 to time slot520.

In some embodiments of the invention, the structure of the CDM pilotwaveform 535 may be a predetermined sequence and associated with anoperation of legacy channel estimation designs. In such embodiments, theTDM pilot waveform 530 may be a new sequence that is constructed toprovide special properties of the resultant combined waveform thatenhance channel estimation and receiver performance. Furthermore, insome embodiments, the resultant combined waveform may enable the use ofefficient channel estimation algorithms at the receiver even in thepresence of the legacy CDM pilot channel.

For example, in some embodiments the special properties of the resultantcombined waveform, during the TDM pilot transmission period, mayinclude:

(i) A cyclic nature of the signal construction, thereby enabling a useof computationally-efficient FFT-based channel estimation algorithms; or

(ii) Low noise degradation factor (NDF) of the resultant combinedsignal, thereby enabling high quality estimation of the radiopropagation channel to be performed (as described later).

It is known that the above-mentioned properties are desirable attributesof a pilot signal. However, it has been recognised by the inventor thatthe presence of a simultaneously-transmitted CDM pilot and also of anyscrambling applied to the composite signal (including the TDM pilot) mayhave the potential to destroy the above-mentioned desirable properties.There is no known mechanism that teaches ways to construct TDM pilotsequences that preserve the above properties, whilst counteracting anypotential action of a coexisting CDM pilot (which may not have beendesigned with the same optimisations in mind).

Additionally, it may be the case that, for legacy reasons, the resultantcomposite signal (including the TDM pilot 530) may be subject to ascrambling sequence that is similarly applied to the data and to the CDMpilot 535. Scrambling is typically applied to distinguish signals fromdiffering transmitters or cells. If applied to the time region of thetransmission of the TDM pilot 530, such scrambling sequences have thepotential to destroy the desirable TDM pilot properties, including thecyclic property. Thus, the inventor has also recognised that there is noknown technique whereby the desirable TDM pilot properties mentionedabove may be preserved, even in the presence of such scrambling of thecomposite signal.

Thus, further embodiments of the invention have been developed toprovide new ways by which the desirable properties of the TDM pilotsequence may be preserved, even in the presence of a scrambling sequencethat is applied to the composite signal. Thereby, embodiments of theinvention may circumvent one or more disadvantages associated with thepresent art where a TDM pilot is to be transmitted together with a CDMpilot for a digital transmission scheme.

It will be understood that in some embodiments the length of the TDMpilot sequence 530 in FIG. 5 will be a power of ‘2’, e.g. 64, 128, 256,512. Longer lengths may be desirable depending on the application.Selecting a sequence of such a length enables efficient channelestimation through the use of, but not limited to, the Discrete FourierTransform (DFT) or Fast Fourier Transform (FFT).

Some embodiments of the invention conform to the general signal patterndescription and construction of FIG. 5. Furthermore, some embodimentsprovide that the CDM pilot may be a pseudo-random binary or quaternaryrandom sequence, such as that used by CPICH in the 3GPP wideband codedivision multiple access (WCDMA) universal mobile telecommunicationsystem (UMTS) frequency division duplex (FDD) system. Such a waveform isnot designed to exhibit periodic properties over the duration of the TDMpilot region, and hence may present an obstacle to retention of thisdesirable periodic property.

The CPICH in the 3GPP WCDMA UMTS FDD system is constructed by modulatingone code from a set of orthogonal channelization spreading codes with aknown sequence. Both the spreading code that is modulated, and thesequence used, are known a-priori to the receiver. Data and/or othersignals (such as for example a code portion of a TDM pilot) aremultiplexed together in the code domain and the resultant compositesignal is subject to a scrambling operation as shown in FIG. 6.

Referring now to FIG. 6, a CDMA signal construction 600 using spreading,code multiplexing and scrambling is illustrated in accordance with someembodiments of the invention. Here, a CDM pilot signal, for example in aform of a CPICH signal, 610 is multiplied with a (CPICH) channelisationcode 615 in multiplier logic 620. Similarly, a plurality of TDM pilotsignals or data 630 are multiplied with a respective channelisation code635, 637, 639, 641 in respective multiplier logic units 645, 647, 649,651. The outputs from each of the multiplier logic units 620, 645, 647,649, 651 are combined in summing logic 625 to produce a constructed CDMAsignal with spreading and code multiplexing 660. The constructed CDMAsignal with spreading and code multiplexing 660 is then multiplied witha scrambling code 665 in multiplier logic 670 to produce compositesignal 675, as illustrated.

In some embodiments, this scrambling code/sequence 665 may be inaccordance with the cell-specific scrambling codes used in the 3GPPWCDMA UMTS system. Such a specific scrambling waveform is not designedto exhibit periodic properties over the duration of the TDM pilotregion, and therefore may present an obstacle to retention of thisdesirable periodic property.

Thus, in some embodiments of the invention, a code portion of a TDMpilot signal may be used such that the scrambled overall combination ofthe code portion of the TDM pilot with the CDM pilot exhibits a cyclicproperty. By arranging that the scrambled overall combination of thecode portion of the TDM pilot and CDM pilot exhibits a cyclic property,the use of computationally-efficient FFT-based channel estimationalgorithms may be employed.

Referring now to FIG. 7, an example of a cyclic construction 700 of acomposite sequence signal 775, for a TDM pilot time region, isillustrated in accordance with one embodiment of the invention. Thecomposite sequence signal 775 is illustrated first as a representationof channelisation spreading codes 710 versus time. Here, the data 725and TDM pilot sequence 730 share the same channelisation spreadingcodes, with the CDM pilot 735 (for example on a CPICH) utilising adifferent set of channelisation spreading codes. Together, the data 725,TDM pilot sequence 730 and CDM pilot 735 form the composite sequencesignal 775.

The composite sequence signal 775 is illustrated secondly as a timedomain representation, whereby the composite sequence signal 775comprises a base sequence component 745 and a cyclic prefix component740. The cyclic prefix component 740 constitutes a replicated 755portion 750 of the base sequence component 745. In one embodiment, theduration of the cyclic prefix component 740 may be intentionally alignedwith the delay spread (dispersion) associated with the radio propagationchannel. In one embodiment, the duration of the cyclic prefix component740 may also occur such that the cyclic prefix component 740 has thesame duration as the base sequence component 745.

In particular, the use of a cyclic structure for a pilot signal mayallow for the use of an efficient and high performance decorrelatingchannel estimation process. Such a scheme may correspond to a maximumlikelihood estimation of the amplitude and phase characteristics of theradio propagation channel.

For example, in the receiver, the preferred filter to estimate theimpulse response h(t) of the radio channel may be a filter thatcorresponds to the signal processing inverse of the pilot time domainsequence p(t), wherein ‘t’ represents a time index. When p(t) has acyclic structure (incorporating the cyclic prefix component concatenatedwith the base sequence component), the application of the preferredinverse filter within the channel estimation processing may beimplemented in the frequency domain via a simple vector divisionoperation. In order to implement the inverse filter in a computationallyefficient manner, a portion of the received signal may be firsttransformed into the frequency domain using a Discrete Fourier Transform(DFT) or a Fast Fourier Transform (FFT), to produce a frequency domainvector representation of the received signal portion, denoted R(f)wherein T denotes a frequency index. The inverse filter may then beapplied by performing a simple element-by-element division operation onR(f). The divisor involved in this element-by-element division operationmay be a vector B_(inv)(f) that may be formed by performing a DFT or FFToperation on the time-domain pilot base sequence b(t). Thiscomputationally efficient inverse filtering process to produce a highquality channel estimate is only possible if the pilot sequence p(t) iscyclic comprising a concatenation of a base sequence b(t) with apre-pended cyclic prefix component itself also formed from a portion ofb(t).

Sequences that are optimised for channel estimation often exhibit lownoise degradation factor (NDF). Effectively, this translates to acondition whereby the power spectral density (PSD) of the sequence(formed via Fourier analysis or similar) is approximately flat inamplitude. Thus, in an embodiment where an inverse filter structure isapplied in the receiver to assist with channel estimation, there islittle enhancement of any noise present on the signal and performance isimproved when compared to a case where the sequence exhibits a powerspectral density that is not flat in amplitude.

Thus, the use of ‘spectrally-flat’ pilot sequences are known in the artto be a desirable attribute to achieve good channel estimation andreceiver performance.

The cyclic prefix component 740 of the pilot is not normally directlyused in the channel estimation process due to the fact that this portionwill typically experience time dispersed interference from precedingdata. Instead, the received portion of the signal during the basesequence component region 745 is normally used as the input to thechannel estimation process. Due to the presence of the cyclic prefixcomponent 740, the received signal portion 750 during the base sequencecomponent 745 is cyclic in structure, and is thus well suited tolow-complexity channel estimation algorithms that utilise FFT-basedprocessing techniques.

The composite signal during the TDM pilot region may be contaminated bya presence of a CDM pilot, which is non-cyclic in structure.Furthermore, the composite signal during the TDM pilot region may bescrambled by a non-cyclic scrambling sequence.

Therefore, in order to preserve the desirable cyclic sequence propertyof the combined signal, the TDM pilot sequence is designed tospecifically counteract the effect of the non-cyclic CDM pilot and ofany non-cyclic scrambling. Furthermore, this is achieved using signalsthat when combined with the CDM pilot, exhibit a low noise degradationfactor for the resultant composite signal.

It will be understood that the non-cyclic CDM pilot and the non-cyclicscrambling may only be non-cyclic over a finite range of time. In oneembodiment, a radio frame of 10 msec is subdivided into slots of length10 msec./15 msec. In this embodiment, each slot contains a TDM pilot.The duration of the CDM pilot and scrambling sequences may be equal toone radio frame and are therefore cyclic at a period equal to the radioframe length of 10 msec. The CDM pilot and scrambling sequences are notnecessarily cyclic over a time period shorter than the radio frameduration. It may therefore be the case that the TDM pilot is also cyclicat a period equal to the radio frame length of 10 msec., although thismay not be the case at the slot length of 10 msec/15 msec, since aunique TDM pilot is required on each slot to preserve the desirableproperties of the combined signal.

Referring now to FIG. 8, an example 800 of a creation of a cyclic TDMpilot region using a non-cyclic CDM pilot sequence 805 and a non-cyclicTDM pilot sequence 810 is illustrated in accordance with embodiments ofthe invention. The non-cyclic CDM pilot sequence 805 and a non-cyclicTDM pilot sequence 810 are combined in summing logic 815 to produce acyclic composite sequence 820. Thus, the composite signal 775 is formedfrom a cyclic composite sequence 820 enveloped by arbitrary sequences825. Again, the cyclic composite sequence signal 820 comprises a basesequence component 745 and a cyclic prefix component 740. The cyclicprefix component 740 constitutes a replicated 755 portion 750 of thebase sequence component 745.

Referring now to FIG. 9, examples of CDM and TDM pilot constructionmethods are illustrated, according to some embodiments of the invention.A skilled artisan will appreciate that the exact generation of the CDMand TDM pilot sequences may involve one or more signal processing stepsthat is/are not explicitly shown in FIG. 9, and thus FIG. 9 provides apotentially simplified version of what may be implemented in practice.

With reference to a first embodiment 910 illustrated in FIG. 9A, apredefined and generally non-cyclic CDM pilot sequence 915 may beapplied to summing logic 925. The CDM pilot sequence 915 may correspondto a scrambling sequence assigned to the transmitter or cell or to someother scrambling sequence. The scrambling sequence is generallynon-cyclic in nature and is compatible with channel estimation methodsused within legacy terminal designs. The CDM pilot sequence 925 issummed together with a TDM pilot sequence 920 in summing logic 925 inorder to produce a composite sequence 928 that exhibits a cyclicproperty suitable for improved channel estimation and for itscomputationally efficient implementation (for example using Fast FourierTransform (FFT)-based algorithms).

In a second embodiment 930 illustrated in FIG. 9B, a scrambling functionmay be implemented, for example using scrambling logic 936, where thescrambling codes are associated with all signals transmitted by theNode-B transmitter (or similar network element in the communicationcell). A CDM pilot sequence 932 is input to the scrambling logic 936.The same scrambling function is also applied to a TDM pilot sequence934. The two outputs from the scrambling logic 936 (corresponding to thescrambled CDM pilot and to the scrambled TDM pilot respectively) aresummed in summing logic 938 in order to produce a composite sequence 940that exhibits a cyclic property suitable for improved channel estimationand for its computationally efficient implementation (for example usingFFT-based algorithms).

A third embodiment 950 illustrated in FIG. 9C is similar to that of FIG.9B, with the exception that the TDM pilot sequence is constructed usinga combination of several constituent sequence fragments 954. Thefragments 954 are encoded and multiplexed into a single signal stream bymultiplexing logic 956. A variety of multiplexing logic modules 956 maybe applied and used as appropriate for specific circumstances. Forexample, the multiplexing logic 956 may consist of a frequencymultiplexer such as a Discrete Fourier Transform (DFT) or a Fast FourierTransform (FFT). Alternatively, the multiplexing logic 956 may consistof a channelization code spreading unit and code multiplexer similar tothat shown in FIG. 6. In general the TDM sequence fragments 954 couldeach be used to modulate one of a set of orthogonal basis functions,such as complex exponentials (in the case of a DFT/FFT implementation)or channelization codes (in a spreading and code multiplexerimplementation). Hence, any set of orthogonal functions may be employedwithin the multiplexing logic 956. The two outputs from the scramblinglogic 952 (corresponding to the scrambled CDM pilot and to thescrambled, multiplexed TDM pilot sequence fragments respectively) aresummed in summing logic 960 in order to produce a composite sequence 962that exhibits a cyclic property suitable for improved channel estimationand for its computationally efficient implementation (for example usingFFT-based algorithms). It is also envisaged in alternative embodimentsthat a use of non-orthogonal functions may also be possible.

A fourth embodiment 970 illustrated in FIG. 9D, is similar to that ofFIG. 9C, with the exception that the multiplexing logic used to combinethe TDM pilot sequence fragments comprises a spreading and codemultiplexing logic 976 as typically used for data transmission in 3GPPWCDMA. Thus, by constructing the TDM pilot sequence using a set of TDMpilot symbol sequence fragments 974 in this manner allows for generationof the TDM pilot sequence using the same signal processing steps thatare used for existing CDMA data transmission. In this way, the impact ofthe introduction of a TDM pilot on legacy CDM-based hardware andsoftware implementations and architectures may be reduced.

The steps of data signal transmission are not explicitly shown in FIG.9. According to the art known in 3GPP WCDMA, the data portion of thesignal may be constructed to be code-division multiplexed with the CDMpilot using orthogonal variable spreading factor (OVSF) codes as shownin FIG. 6. The data bits are modulated onto data symbols, which may takethe form of a finite alphabet of symbols. Such modulation schemes arewell known and comprise for example BPSK, QPSK, 8-PSK, 16-QAM, or64-QAM. The data portion of the signal is typically transmitted during atime portion of the time slot that does not overlap with the TDM pilotregion of the time slot, as is evident from FIG. 7.

In general, it is envisaged in other embodiments that the TDM pilotsequence does not need to conform to a construction using OVSF (or otherpredefined) spreading codes, nor to a particular finite alphabet ofmodulation symbols.

However, it may be desirable in some embodiments to construct the TDMpilot sequence using OVSF codes and/or using symbols from a finitealphabet, such that it may be compatible with the same data transmitmodulation architecture of the transmitter hardware. The use of a finitealphabet of symbols, and of a particular construction method for the TDMpilot sequence, represents a variety of signal or sequence constraints,commonly known as quantisation.

It will also be appreciated that other quantisation methods may also beapplied to the TDM pilot sequence or to its associated sequencefragments. It is envisaged that these may include, for example,constant-amplitude signal construction with arbitrary (unconstrainedphase), phase shift keying modulation (8-PSK, 16-PSK, 32-PSK), andarbitrary quadrature amplitude modulation such as 64-QAM or 256-QAM.

At a receiver, various channel estimation and data recovery techniquesmay be employed. In some embodiments there may exist either a CDMchannel estimator component, or a TDM channel estimator component. Insome embodiments, both a CDM and a TDM channel estimator component maybe used, in combination, to improve the accuracy and quality of thechannel estimate.

Thus, in one embodiment, the CDM channel estimator component may bearranged to analyse a structure of the received composite signal duringthe timeslot in order to search for a presence of the CDM pilotsequence, (such as for example, the CPICH in 3GPP WCDMA systems). TheCDM channel estimator component may then generate a first estimate ofthe radio channel parameters.

The TDM channel estimator component may analyse a structure of thereceived composite signal during the TDM pilot region of the timeslot,in order to search for a presence of the TDM pilot sequence only.Alternatively, in one embodiment, the TDM channel estimator componentmay search for a presence of a composite sequence during the same timeregion. Advantageously, when the composite signal exhibits acyclically-prefixed structure, such as that shown in FIG. 9,computationally-efficient structures may be employed in order to returna second estimate of the radio channel parameters. Suchcomputationally-efficient methods may include, for example, the use ofDFT or FFT transforms that rely upon the cyclic structure of thereceived signal during the TDM pilot region.

In yet further embodiments, it is also envisaged that data may berecovered using only the first or the second estimates of the radiochannel. Optionally, the first estimate may be combined with the secondestimate in order to form a combined estimate, which may improve thequality of the channel estimation and may improve the performance of theradio link.

Referring now to FIG. 10, an example of a receiver architecture 1000,adapted to employ a combined pilot signal transmission scheme, isillustrated and comprises logic arranged to receive composite receivedsignals 1005. The composite received signals 1005 comprise at least aTDM pilot portion 1010, as described previously. A first compositereceived signal 1005 is input to a CDM channel estimator 1015, whichoutputs a first estimate of channel parameters 1020. A second compositereceived signal 1005 is input to a separate TDM channel estimator 1025,which outputs a second estimate of channel parameters 1030. Asillustrated in the example receiver architecture 1000, CDM-derivedchannel estimates 1020 and/or TDM-derived channel estimates 1030 may beused in subsequent signal processing steps within the receiver.

For example, as illustrated, the CDM-derived channel estimates 1020and/or TDM-derived channel estimates 1030 may be combined in combininglogic 1035 and input to equalisation and data recovery logic 1045. Theequalisation and data recovery logic 1045 also receives signals from thedata portion of the received time slot 1040 and outputs the recoveredcommunication data 1050. In this manner, the CDM-derived channelestimates 1020 and/or TDM-derived channel estimates 1030 may be used tocombat any effect of time dispersion within the radio propagationchannel, using suitable equalisation structures arranged in equalisationand data recovery logic 1045, as are well known in the art and thus notdescribed further here.

In a further example embodiment, it is also envisaged that the CDM pilotsequences and/or TOM pilot sequences may be used within the receiver totrain equaliser structures that combat the effects of time dispersionwithin the radio channel, notably without involving an explicit channelestimation stage. An example of such an adaptive equaliser architecture1100 is illustrated in FIG. 11.

As illustrated in the adaptive equaliser architecture 1100 of FIG. 11, alocal replica of the known transmitted pilot sequences 1105 is comparedwith an equalised pilot signal 1140 output from an adaptive equaliser1120. The adaptive equaliser 1120 receives a composite received signal1125. The received pilots are extracted from the composite equalisedoutput using the pilot demultiplexing logic 1115. The resultant error1135 between the local replica 1105 and the equalised pilot 1140 is usedto intelligently adjust the coefficients 1130 used within the equaliserstructure for use in a subsequent time period. Thus, it is envisagedthat such adaptive equaliser structures may also use a TDM pilotcomponent or CDM pilot component (or combination of both a TDM pilot andCDM pilot) as an input to the adaptation process.

The above-mentioned embodiments of the invention describe how aspecially-designed TDM pilot sequence may be superimposed onto anon-cyclic portion of a CDM pilot signal, in order to produce acomposite sequence with desirable cyclic properties.

It is envisaged that many TDM pilot sequences may be used to achievethis goal. However, for anything other than very short sequence lengths,the number of possible sequences to search becomes quickly unmanageable.For example, even if the number of possible TDM pilot sequence isconstrained to the simplest possible (i.e. binary) symbol alphabet,there are 2¹²⁸=3.4×10³⁸ possible sequences to search. For non-binarysymbol alphabets, this rapidly increases to a much larger set ofpossible sequences.

As previously mentioned, it is also important that the base sequence ofthe TDM pilot has relatively flat frequency content, in order tomaintain a low NDF when channel estimation is performed using adecorrelating channel estimator. Therefore, an intelligent sequencesearch process may be advantageously employed, in order to findsequences from a potentially very large set of possible sequences thatalso satisfy the low NDF criterion.

Referring now to FIG. 12, an example of a sequence search process 1200that may be employed in some embodiments of the invention isillustrated. For example, the sequence search process 1200 is describedfor a scenario whereby the cyclic prefix duration of the combined pilotsequence is equal to the base sequence duration. The length of the basesequence is denoted as ‘L’ (thus the length of the cyclic prefixduration is also equal to ‘L’).

The search process 1200 is an iterative process. Referring to FIG. 12,li starts by randomly constructing a desired flat frequency domaincontent X(f) wherein ‘f’ is a frequency index 1 . . . L (this wouldsatisfy the low NDF requirement). Note that the term ‘flat’ implies thatthe powers of a signal within each sub-band of a frequency range areapproximately equal. An inverse fourier transform (e.g. IFFT) isperformed on this desired output X(f) in order to derive the time domainsequence x(t) (also of length L) that would result in the flat frequencydomain content of X(f).

If the overall pilot sequence is cyclic, comprising a base sequence oflength L and a cyclic prefix also of length L, then the desired timedomain signal for the total pilot signal is the concatenation of x(t)with itself.

For a scrambled non-cyclic CDM pilot component c(t) of length 2L, thismay be considered as the concatenation of two different scrambled CDMpilot sequences c₁(t) and c₂(t), each of length L.

The signal x(t) of length L therefore comprises the summation of theideal first half of the scrambled TDM pilot sequence z₁(t) with thefirst half of the non-cyclic scrambled CDM pilot sequence c₁(t), orequivalently the summation of the ideal second half of the scrambled TDMpilot sequence z₂(t) with the second half of the non-cyclic scrambledCDM pilot sequence c₂(t). Thus, z₁(t) can be derived as x(t)−c₁(t) andz₂(t) can be derived as x(t)−c₂(t).

In this example, let us consider that the TDM pilot sequence halvesz₁(t) and z₂(t) are each constructed using a modulated and scrambled setof OVSF channelisation codes as shown in FIG. 9D. Each channelisationcode is modulated using a TDM sequence ‘fragment’, again as shown inFIG. 9D. In order to derive the TDM pilot sequence fragments, thesignals z₁(t) and z₂(t) are first descrambled by thetransmitter-specific scrambling sequences for each half s₁(t) and s₂(t),and are optionally then demultipexed into their constituent orthogonalfunction components using a demultiplexing logic unit. The outputs ofthe two demultiplexing logic units are denoted w_(u, 1) and w_(u, 2) forthe u^(th) orthogonal basis function (u=1 . . . U) and for the 1^(st)and 2^(nd) signal halves respectively (corresponding to the cyclicprefix and base sequence halves of the signal). For each of the twobranches, the set of U outputs are then quantised to a particular symbolalphabet to form Q_(u, 1) and Q_(u, 2).

The demultiplexing logic units may, in general, be constructed as a bankof filters each matched to an individual member of the set of Uorthogonal functions. In the context of the example corresponding toFIG. 9D, the demultiplexing logic unit may consist of a set of CDMAchannelisation code matched filters. If the TDM pilot sequence is notcomprised of a combination of orthogonal basis functions, thedemultiplexing stage may be omitted.

Following the symbol quantisation stage, the signal deconstructionportion of the iteration is complete. The signals for each of the twobranches may then be reconstructed following the same (but inverse)signal processing steps to derive two reconstructed time domain signalsy₁(t) and y₂(t), each of length L. A DFT or FFT of y₁(t) and y₂(t) istaken in order to evaluate the frequency domain content of the derivedsignals, producing Y₁(f) and Y₂(f). On, say, even iterations, the angleof the signal Y₁(t) may be taken (with its amplitude set to unity) andmay be used as the seed X(f) for the next iteration. On odd iterations,for example, the angle of the signal Y₂(t) may be taken (again with itsamplitude set to unity) and may be used as the seed X(f) for the nextiteration.

This process may be repeated until y₁(t) and y₂(t) converge to the same(or a similar) sequence. When this is achieved, the overall compositepilot sequence formed from the concatenation of y₁(t) with y₂(t) and oflength 2L is approximately cyclic (thus, the base sequence component andcyclic prefix component are constructed from a common sequence). Thiscomposite sequence is formed using the derived TDM pilot sequencefragments Q_(u, 1) and Q_(u, 2) for the cyclic prefix component and thebase sequence component respectively. Q_(u, 1) and Q_(u, 2) aregenerally not the same but form similar sequences when scrambled withtheir respective scrambling sequence portions (s₁(t) and s₂(t)) and whensubsequently combined with their respective CDM pilot portions c₁(t) andc₂(t). The signals y₁(t) and y₂(t) may be nominally the same and bothshould exhibit the desired low noise degradation property (i.e. theyhave relatively ‘flat’ frequency domain content).

Referring now to FIG. 13, there is illustrated a typical computingsystem 1300 that may be employed to implement signal processingfunctionality in embodiments of the invention. Computing systems of thistype may be used in access points and wireless communication units.Those skilled in the relevant art will also recognize how to implementthe invention using other computer systems or architectures. Computingsystem 1300 may represent, for example, a desktop, laptop or notebookcomputer, hand-held computing device (PDA, cell phone, palmtop, etc.),mainframe, server, client, or any other type of special or generalpurpose computing device as may be desirable or appropriate for a givenapplication or environment. Computing system 1300 can include one ormore processors, such as a processor 1304. Processor 1304 can beimplemented using a general or special-purpose processing engine suchas, for example, a microprocessor, microcontroller or other controllogic. In this example, processor 1304 is connected to a bus 1302 orother communications medium.

Computing system 1300 can also include a main memory 1308, such asrandom access memory (RAM) or other dynamic memory, for storinginformation and instructions to be executed by processor 1304. Mainmemory 1308 also may be used for storing temporary variables or otherintermediate information during execution of instructions to be executedby processor 1304. Computing system 1300 may likewise include a readonly memory (ROM) or other static storage device coupled to bus 1302 forstoring static information and instructions for processor 1304.

The computing system 1300 may also include information storage system1310, which may include, for example, a media drive 1312 and a removablestorage interface 1320. The media drive 1312 may include a drive orother mechanism to support fixed or removable storage media, such as ahard disk drive, a floppy disk drive, a magnetic tape drive, an opticaldisk drive, a compact disc (CD) or digital video drive (DVD) read orwrite drive (R or RW), or other removable or fixed media drive. Storagemedia 1318 may include, for example, a hard disk, floppy disk, magnetictape, optical disk, CD or DVD, or other fixed or removable medium thatis read by and written to by media drive 1312. As these examplesillustrate, the storage media 1318 may include a computer-readablestorage medium having particular computer software or data storedtherein.

In alternative embodiments, information storage system 1310 may includeother similar components for allowing computer programs or otherinstructions or data to be loaded into computing system 1300. Suchcomponents may include, for example, a removable storage unit 1322 andan interface 1320, such as a program cartridge and cartridge interface,a removable memory (for example, a flash memory or other removablememory module) and memory slot, and other removable storage units 1322and interfaces 1320 that allow software and data to be transferred fromthe removable storage unit 1318 to computing system 1300. Computingsystem 1300 can also include a communications interface 1324.

Communications interface 1324 can be used to allow software and data tobe transferred between computing system 1300 and external devices.Examples of communications interface 1324 can include a modem, a networkinterface (such as an Ethernet or other NIC card), a communications port(such as for example, a universal serial bus (USB) port), a PCMCIA slotand card, etc. Software and data transferred via communicationsinterface 1324 are in the form of signals which can be electronic,electromagnetic, and optical or other signals capable of being receivedby communications interface 1324. These signals are provided tocommunications interface 1324 via a channel 1328. This channel 1328 maycarry signals and may be implemented using a wireless medium, wire orcable, fiber optics, or other communications medium. Some examples of achannel include a phone line, a cellular phone link, an RF link, anetwork interface, a local or wide area network, and othercommunications channels.

In this document, the terms ‘computer program product’‘computer-readable medium’ and the like may be used generally to referto media such as, for example, memory 1308, storage device 1318, orstorage unit 1322. These and other forms of computer-readable media maystore one or more instructions for use by processor 1304, to cause theprocessor to perform specified operations. Such instructions, generallyreferred to as ‘computer program code’ (which may be grouped in the formof computer programs or other groupings), when executed, enable thecomputing system 1300 to perform functions of embodiments of the presentinvention. Note that the code may directly cause the processor toperform specified operations, be compiled to do so, and/or be combinedwith other software, hardware, and/or firmware elements (e.g., librariesfor performing standard functions) to do so.

In an embodiment where the elements are implemented using software, thesoftware may be stored in a computer-readable medium and loaded intocomputing system 1300 using, for example, removable storage drive 1322,drive 1312 or communications interface 1324. The control logic (in thisexample, software instructions or computer program code), when executedby the processor 1304, causes the processor 1304 to perform thefunctions of the invention as described herein.

It will be appreciated that, for clarity purposes, the above descriptionhas described embodiments of the invention with reference to differentfunctional units and processors. However, it will be apparent that anysuitable distribution of functionality between different functionalunits or processors, for example with respect to the broadcast modelogic or management logic, may be used without detracting from theinvention. For example, functionality illustrated to be performed byseparate processors or controllers may be performed by the sameprocessor or controller. Hence, references to specific functional unitsare only to be seen as references to suitable means for providing thedescribed functionality, rather than indicative of a strict logical orphysical structure or organization.

Aspects of the invention may be implemented in any suitable formincluding hardware, software, firmware or any combination of these. Theinvention may optionally be implemented, at least partly, as computersoftware running on one or more data processors and/or digital signalprocessors. Thus, the elements and components of an embodiment of theinvention may be physically, functionally and logically implemented inany suitable way. Indeed, the functionality may be implemented in asingle unit, in a plurality of units or as part of other functionalunits.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.In the claims, the term ‘comprising’ does not exclude the presence ofother elements or steps.

Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by, for example, a singleunit or processor. Additionally, although individual features may beincluded in different claims, these may possibly be advantageouslycombined, and the inclusion in different claims does not imply that acombination of features is not feasible and/or advantageous. Also, theinclusion of a feature in one category of claims does not imply alimitation to this category, but rather indicates that the feature isequally applicable to other claim categories, as appropriate.

Furthermore, the order of features in the claims does not imply anyspecific order in which the features must be performed and in particularthe order of individual steps in a method claim does not imply that thesteps must be performed in this order. Rather, the steps may beperformed in any suitable order. In addition, singular references do notexclude a plurality. Thus, references to “a”, “an”, “first”, “second”etc. do not preclude a plurality.

The invention claimed is:
 1. A method performed by a base station (BS),the method comprising: generating, by the BS, a first pilot sequence,wherein the first pilot sequence is a code division multiplexing (CDM)pilot sequence and is spread with a first channelization code;generating, by the BS, a second pilot sequence, wherein the second pilotsequence is a plurality of time division multiplexing (TDM) pilotsequences including a cyclic prefix and is spread with a secondchannelization code; performing, by the BS, a scrambling operation onthe first pilot sequence and the second pilot sequence duringsubstantially overlapping plurality of time slots; transmitting, by theBS, data, the first pilot sequence and the second pilot sequence duringthe substantially overlapping plurality of time slots as a compositesignal, wherein the composite signal is within a designated orpre-defined time region such that the first pilot sequence hasnon-cyclic properties over the designated or pre-defined time region;wherein the second pilot sequence is composed such that the compositesignal has a cyclic prefix time domain structure within the designatedor pre-defined time region; and wherein the cyclic prefix includes areplicated portion of a base portion of the composite signal.
 2. Themethod of claim 1 further comprising: modulating, by the BS, thecomposite signal using a finite symbol alphabet or constant-amplitudesymbols.
 3. The method of claim 1, wherein the first pilot sequence is aportion of a common pilot channel of a third generation partnershipproject (3GPP) wideband code division multiple access (WCDMA) universalmobile telecommunication standard (UMTS) system.
 4. The method of claim1, wherein the first pilot sequence or the second pilot sequence enablesa wireless communication unit to demodulate a data symbol.
 5. A basestation (BS) comprising: a processor configured to generate a firstpilot sequence, wherein the first pilot sequence is a code divisionmultiplexing (CDM) pilot sequence and is spread with a firstchannelization code; the processor further configured to generate asecond pilot sequence, wherein the second pilot sequence is a pluralityof time division multiplexing (TDM) pilot sequences including a cyclicprefix and is spread with a second channelization code; the processorfurther configured to perform a scrambling operation on the first pilotsequence and the second pilot sequence during substantially overlappingplurality of time slots; a transmitter configured to transmit, data, thefirst pilot sequence and the second pilot sequence during thesubstantially overlapping plurality of time slots as a composite signal,wherein the composite signal is within a designated or pre-defined timeregion such that the first pilot sequence has non-cyclic properties overthe designated or pre-defined time region; wherein the second pilotsequence is composed such that the composite signal has a cyclic prefixtime domain structure within the designated or pre-defined time region;and wherein the cyclic prefix includes a replicated portion of a baseportion of the composite signal.
 6. The BS of claim 5 furthercomprising: the processor further configured to modulate the compositesignal using a finite symbol alphabet or constant-amplitude symbols. 7.The BS of claim 5, wherein the first pilot sequence is a portion of acommon pilot channel of a third generation partnership project (3GPP)wideband code division multiple access (WCDMA) universal mobiletelecommunication standard (UMTS) system.
 8. The BS of claim 5, whereinthe first pilot sequence or the second pilot sequence enables a wirelesscommunication unit to demodulate a data symbol.
 9. A method performed bya wireless communication unit, the method comprising: receiving, by thewireless communications unit on substantially overlapping plurality oftime slots, a composite signal, wherein the composite signal includesdata symbols, a first pilot sequence, wherein the first pilot sequenceis a code division multiplexing (CDM) pilot sequence and is spread witha first channelization code, and a second pilot sequence, wherein thesecond pilot sequence is a plurality of time division multiplexing (TDM)pilot sequences including a cyclic prefix and is spread with a secondchannelization code; wherein the composite signal is received within adesignated or pre-defined time region such that the first pilot sequencehas non-cyclic properties over the designated or pre-defined timeregion; wherein the second pilot sequence is composed such that thecomposite signal has a cyclic prefix time domain structure within thedesignated or pre-defined time region; wherein the cyclic prefixincludes a replicated portion of a base portion of the composite signal;descrambling, by the wireless communication unit, the composite signalwith a non-cyclic scrambling code; and demodulating, by the wirelesscommunications unit, the composite signal to recover the data symbolsbased on the first pilot sequence or the second pilot sequence.
 10. Themethod of claim 9, wherein the first pilot sequence is a portion of acommon pilot channel of a third generation partnership project (3GPP)wideband code division multiple access (WCDMA) universal mobiletelecommunication standard (UMTS) system.
 11. A wireless communicationsunit comprising: a receiver configured to receive, on substantiallyoverlapping plurality of time slots, a composite signal, wherein thecomposite signal includes data symbols, a first pilot sequence, whereinthe first pilot sequence is a code division multiplexing (CDM) pilotsequence and is spread with a first channelization code, and a secondpilot sequence, wherein the second pilot sequence is a plurality of timedivision multiplexing (TDM) pilot sequences including a cyclic prefixand is spread with a second channelization code; wherein the compositesignal is received within a designated or pre-defined time region suchthat the first pilot sequence has non-cyclic properties over thedesignated or pre-defined time region; wherein the second pilot sequenceis composed such that the composite signal has a cyclic prefix timedomain structure within the designated or pre-defined time region;wherein the cyclic prefix includes a replicated portion of a baseportion of the composite signal; a processor configured to descramblethe composite signal with a non-cyclic scrambling code; and theprocessor configured to demodulate the composite signal to recover thedata symbols based on the first pilot sequence or the second pilotsequence.
 12. The wireless communications unit of claim 11, wherein thefirst pilot sequence is a portion of a common pilot channel of a thirdgeneration partnership project (3GPP) wideband code division multipleaccess (WCDMA) universal mobile telecommunication standard (UMTS)system.